The operating performance of a semiconductor device having active components disposed therein is frequently degraded by dangling bonds which occur primarily at the surface of the device or at vacancies, micropores, dislocations, and the like which are incorporated into the device. The dangling bonds are responsible for states in the energy gap which affect the level of surface recombination. In addition, the performance of a silicon solar cell is often degraded by the highly doped surface region which causes recombinations of some of the electrons and holes, thus preventing the electrons generated by the solar photons from being collected by the elecrtrodes of the solar cell.
Hydrogenated amorphous silicon fabricated by a glow-discharge and devices fabricated therefrom are described in U.S. Pat. No. 4,064,521, incorporated herein by reference. Amorphous silicon, and specifically hydrogenated amorphous silicon fabricated by a glow-discharge, has a band gap energy which is greater than the band gap energy of crystalline silicon. Typically, hydrogenated amorphous silicon has a band gap energy of about 1.7 eV whereas crystalline silicon has a band gap energy of about 1.1 eV. The larger band gap energy of the material at the surface presents a barrier to the minority carriers and therefore reduces loss of the minority carriers through recombination processes at the surface. The glow-discharge produced hydrogenated amorphous silicon contains approximately 18 to 50 atomic percent of hydrogen, as has been shown by J. I. Pankove et al. in Appl. Phys. Letters, Vol. 31 (1977), pg. 450. The hydrogen passivates the dangling bonds in the silicon material. Heating the amorphous silicon in a vacuum causes the hydrogen to evolve, which causes a deterioration in the properties of the hydrogenated amorphous silicon. Thus the hydrogen has been shown to be an effective passivating element for amorphous silicon.
A method of passivating a semiconductor device is described in U.S. Pat. No. 4,113,514, issued Sept. 12, 1978 and incorporated by reference herein. The method comprises exposing the silicon semiconductor device to atomic hydrogen at a temperature lower than about 450.degree.; subsequently, such a device must be kept below this dehydrogenation temperature, otherwise the hydrogen escapes the dangling bonds. The passivation technique can also be applied to crystalline silicon PN junction devices by applying a layer of hydrogenated amorphous silicon on the surface of the semiconductor device. However, upon heating, such layers of hydrogenated amorphous silicon can blister and flake off, thus exposing the crystalline silicon to the ambient environment.
U.S. Pat. No. 4,224,084 entitled METHOD AND STRUCTURE FOR PASSIVATING A SEMICONDUCTOR DEVICE, issued on Apr. 23, 1980, to J. I. Pankove and incorporated herein by reference, provides a technique for neutralizing the dangling bonds in a semiconductor device. The neutralization of the dangling bonds passivates the device and reduces the number of recombination-generation centers in the device. The technique provides for an adherent passivation which can withstand heating up to 700.degree. C. The technique basically involves the amorphization of the surface of crystalline silicon followed by the exposure of the amorphized surface to atomic hydrogen, thereby creating a device of graded crystallinity with a large concentration of hydrogen at the surface of the device.
Although the region of graded crystallinity incorporating atomic hydrogen saturates the dangling bonds, the surface region is not single crystal silicon. It would be desirable to have a single crystal silicon material having a graded band gap due to the incorporation of atomic hydrogen therein, and a method of preparing same.